The present invention relates generally to the field of chemical agent detection. More particularly, the present invention provides a method and apparatus for visually and/or colorimetrically monitoring carbon dioxide (CO2) levels in a gas mixture.
The detection and measurement of chemical agents has numerous applications, including industrial, military, medical, and municipal gas detection. One such application, lying in the field of biomedical instrumentation, is the identification and quantification of expired gases (as from a patient under general anesthesia). It is important for an anesthesiologist to know the concentrations of certain gaseous compounds (including carbon dioxide) in the surgical patient""s expired airstream. This is due to the fact that the concentrations in the airstream at the end of the exhaled breath (end-tidal) are indicative of their concentrations in the blood.
Because of the rapidly changing carbon dioxide concentration in an exhaled breath stream, a suitable detector must be almost instantaneously reversible and have a rapid response.
Accordingly, the present invention provides methods and apparatus related to use of a reversible color detector to rapidly identify the presence and concentration of electrophiles, such as carbon dioxide (CO2) gas, in gas mixtures at levels of approximately 1% to 5% and higher. The carbon dioxide detection involves a substantially instantaneous and reversible color change (that is, a color change occurring in less than 1 second) of a primed substrate that has been coated with a coating composition. For example, if a coated primed substrate detector is positioned in an endotracheal tube during surgery, carbon dioxide in the exhaled breath instantaneously turns the detector progressively from green-blue to shades of lighter green to yellow, while inhalation substantially reverses the color change to return the detector to green-blue.
The coating composition of the present invention comprises an effective amount of organic binder, a color indicator, an activator, a humectant, and an effective amount of filler/pigment. The organic binder comprises ethyl cellulose and the filler/pigment comprises calcium carbonate and amorphous silica. The color indicator preferably comprises bromothymol blue and bromothymol purple (preferably in about a 2:1 ratio). Significant deviation from the 2:1 ratio results in reduced sensitivity to carbon dioxide, reduced intensity of color changes and/or decreased reproducibility of color change in the presence of carbon dioxide. The activator comprises ether amine, preferably dodecyl ether amine and/or tetradecyl ether amine. No other primary or secondary alkyl amine or aromatic amine tested provided sufficient color variance with exposure to changing concentrations of carbon dioxide, combined with reversal of color change when the carbon dioxide was removed. Applying this coating composition to a primed substrate such as to the indicator substrate described herein below, or to (commercially available) pH Hydrion paper (which, when so coated, responds to gaseous carbon dioxide in a manner similar to that of a coated indicator substrate), allows detection of electrophiles of the class comprising carbon dioxide by color change of the coated primed substrate. Accordingly, the method for detecting electrophiles of the class comprising carbon dioxide comprises the steps of coating a primed substrate with a coating composition (as described herein) and sensing a color change of the coated primed substrate. Thus, a coated primed substrate provides a detector apparatus for detecting gaseous electrophiles of the class comprising carbon dioxide.
The coating composition of the present invention may be made by dissolving ethyl cellulose (binder) in acetone and then, in a high speed/high shear mixer, adding to this solution the color indicator, activator, humectant and filler/pigment. Grinding and blending in the mixer preferably continues until the composition is substantially homogeneous. The resulting coating composition is thereafter preferably stored in a closed container and protected from direct sunlight and evaporation of water until ready for application to a primed substrate. A coated primed substrate (as described above) is dried as described below, whereupon it becomes sensitive to carbon dioxide.
The rapid, reversible and quantitative response of detectors of the present invention to carbon dioxide results from the combination of a layer of dyes and indicators (the coating composition) applied over a primed substrate. The substrate is primed with a different set of dyes and the result is a synergism between the primed substrate and the coating composition that provides the desired color changes in the presence of varying concentrations of carbon dioxide gas. Whereas certain other detectors provide a color response which is dependent on time of exposure to carbon dioxide, color response of the present detector is substantially instantaneous (with virtually no further color change after the first few seconds of exposure).